This invention relates to a method for the production of silicon carbide with automatic magnetic separation of a high grade fraction.
Silicon carbide, SiC, is produced by reacting silica with carbon, usually at temperatures of approximately 2000.degree. C., according to the following reactions: ##EQU1## At temperatures above 2000.degree. C., consolidation and recrystallization of gaseous sub-species of silicon carbide take place.
Commercial production of silicon carbide, in the prior art, is commonly carried out in an Acheson furnace, as shown in FIG. 1, in which an electric current is passed through the reactants to form silicon carbide. Silicon carbide has also been produced in other types of furnaces, including vertical shaft furnaces and rotary kilns, in which passing an electrical charge through the silicon carbide is not the method utilized. In these types of furnaces, the heat source may be external to the reactants. Such furnaces generally produce a less pure silicon carbide product.
The most prevalent method for making silicon carbide, today, is almost identical to that described in the 1893 Acheson patent, U.S. Pat. No. 492,767. The cross section of an Acheson-type furnace is usually trapezoidal, as illustrated in FIG. 2, or it may be semicircular. The furnace may have removable firebrick sidewalls or sections which contain the reactants. These furnace walls may be straight, curved, or tapered. Generally, the furnaces are approximately 20 to 200 feet in length, 10 to 20 feet in width, and 6 to 20 feet in height. The furnace may be straight in its length dimension or may be circular, horse-shoe shaped, or other various configurations. At each end of the furnace are rectangular or cylindrical-shaped graphite or soderberg electrodes, which are positioned near the center of the furnace head, as illustrated in FIG. 1. The furnace is charged with a mixture of approximately 45% carbon by weight, generally in the form of coke or anthracite coal, and 55% silica sand, SiO.sub.2, by weight. (Throughout the specification and claims, all percentages are by weight, unless otherwise indicated.) After the furnace is half full, a core of petroleum coke and/or graphite is placed in the center, connecting the electrodes. The purpose of the graphite core is to serve as a conductor between the electrodes to generate the high temperatures necessary for the initial silicon carbide formation. Further silica sand and carbon charge is then placed over the electrode core to fill the furnace. Sawdust and coarse sand are often a part of the furnace charge to promote circulation of the reacting gases and to aid in venting the carbon monoxide gas which is formed during the reaction.
The furnace is heated by applying power to the electrodes for approximately 30 to 360 hours. The voltage required is approximately 200 to 1200 volts, and the current requirements are approximately 5000 to 65,000 amperes. The total power consumption normally ranges from 2.7 to 3.6 kwh per pound of silicon carbide product. Silicon carbide is formed by the reaction of the silica sand with the coke or graphite. This reaction first occurs around the graphite core, and then proceeds outward to eventually form a large "cylinder." As used throughout the specification and claims, the term "cylinder" refers to a silicon carbide furnace product or materials, regardless of its cross-sectional shape, e.g. circular, elliptical, etc. Moreover, the "cylinder" may come from a furnace that is, in plan view, linear, circular or horse shoe shaped. Silicon carbide is a conductor of electricity. As the mixture around the furnace core becomes converted to silicon carbide, the cylinder begins to conduct some of the electrical current, which necessitates a downward adjustment in the applied voltage to limit the power input. Some experts claim that during production of silicon carbide, the furnace temperature rises to a maximum of approximately 2500.degree. C. at the core and then decreases to a nearly constant temperature of approximately 2040.degree. C. Silicon carbide can form at lower temperatures, but the product is a cubic form of silicon carbide (beta silicon carbide) which is generally unsuitable for abrasives purposes because the crystals are too small. The most desirable form of silicon carbide is large crystal alpha silicon carbide (a hexagonal form of silicon carbide), which is formed from beta silicon carbide at temperatures above about 1950.degree. C. The color of silicon carbide varies depending on the purity of the furnace reactants; higher purity reactants produce silicon carbide which tends to be more pure and light green in color, while less pure reactants produce silicon carbide which is blank in color.
As the furnace product cools, the unreacted mix is removed from the silicon carbide furnace product. The resulting furnace cylinder is approximately cylindrical or oval shaped and has three silicon carbide product zones, as illustrated in FIG. 2:
(1) Zone 1, known in the art as #1 Black, first grade, or high grade silicon carbide, contains approximately 65% to 85% by weight of the total furnace cylinder, depending on total impurity content. This zone contains the most pure silicon carbide (approximately 95-99% SiC by weight). This zone comprises coarse crystal, non-porous silicon carbide, and is the most desirable silicon carbide product. However, at the outer layers of this zone, a significant concentration of ferromagnetic elements such as iron may be present. The first grade silicon carbide zone cylinder wall thickness is approximately 4-48 inches thick, the range being a function of furnace size, heating time and total impurity content.
(2) Zone 2, known in the art as firesand, metallurgical, or second grade silicon carbide, comprises fine crystal, porous silicon carbide agglomerated particles. This zone is approximately 10% to 25% by weight of the total furnace cylinder, and contains approximately 92% SiC by weight. This zone may contain significant quantities of ferromagnetic elements such as iron. The concentration of such iron may decrease across the layer in relation to the furnace temperature profile during firing. This layer of silicon carbide may be recycled into subsequent furnace mixes, used as an additive in iron and steelmaking, or used to produce refractories. The second grade silicon carbide zone cylinder wall thickness is approximately 2-12 inches thick.
(3) Zone 3, known in the art as the crust, comprises partially reacted particles. This zone is approximately 5% to 10% by weight of the total furnace cylinder, and contains approximately 60% SiC by weight. The crust contains a considerable amount of silicon carbide, but is unsuitable for abrasive purposes. Silicon carbide in the crust varies in concentration throughout the crust thickness, and nowhere is it well crystallized. The contaminants in the crust are high amounts of silica, calcium, carbon, aluminum and small amounts of iron. The crust zone cylinder wall thickness is approximately 1/2-3 inches thick.
The central core of the silicon carbide furnace cylinder is highly porous graphite (also shown in FIG. 2). Surrounding the silicon carbide crust layer is unreacted mix (also shown in FIG. 2), which can be easily separated from the cylinder and recycled. The unreacted mix may contain a substantial fraction of silicon carbide (up to 30% by weight), which will be consolidated into a cylinder if it is in the reaction zone during subsequent furnace cycles.
After the silicon carbide furnace cylinder is removed from the furnace, the outer crust of the cylinder can be scraped away from the cylinder with a hoe-type device. A hydraulic grab is generally used to split the cylinder into large sections and to expose the porous graphite core. The graphite core is removed by hand, or mechanically with a crane or vacuum device. Additional removal of graphite is accomplished by vacuuming or brushing the pieces removed from the furnace.
The silicon carbide containing zones of the furnace cylinder are not easily separated from each other. The zones are usually separated in the prior art by using hand-held pneumatic spades or jackhammers, as illustrated in FIG. 3. Typically, one person can separate 1400 tons of first grade silicon carbide per year, manually. Hand separation is strenuous, noisy and dusty, so productivity is low. The sorting of the products is done solely by visual appearance; as the sorter chisels the outer layer off a piece of the cylinder, he or she must decide how thick a layer must be removed and into which group the respective pieces must be placed. One problem with hand separation is that some of the best product (first grade silicon carbide) may be lost to the lower grades of silicon carbide product. On the other hand, some of the lower grade silicon carbide may get mixed in with the first grade silicon carbide, which may be detrimental to the end use of the first grade final product. The hand separation and sorting process is thus inefficient, time consuming, inexact, and results in high process costs due to the high amount of labor involved.
After sorting, the masses or lumps of silicon carbide materials are further crushed, washed, dried, size-classified, magnetically treated to remove iron contamination resulting from the crushing mills, and often treated with acid or alkali to improve purity.
There are other prior art methods of separating the silicon carbide furnace materials, based on mechanical, chemical and electrical properties, but these have tended to be more expensive and less reliable than hand separation. Shaking tables or air tables have been utilized to separate silicon carbide particles based on different specific gravities. However, the first grade silicon carbide crystals and agglomerate silicon carbide particles all fall within a narrow range of specific gravities of approximately 2.5-3.2 g/cm.sup.2. Thus, mechanical means of separation are generally ineffective for separating silicon carbide furnace materials.
Another prior art method of separating the various silicon carbide furnace materials is the use of sink-float or heavy medium liquid techniques. Most of these high density liquids, however, are polyhalogenated materials, which are expensive, potentially hazardous, and non-biodegradable. In addition, the silicon carbide materials must generally be crushed to a very fine size and preferably subjected to an elaborate froth flotation process prior to the heavy medium liquid separation step, which are both costly steps.
Another separation technique in the prior art utilizes the electrical conductivity of the silicon carbide materials. Conductive grains of silicon can be separated from non-conductive grains (sand and non-conductive silicon carbide) by electrostatic or high tension separators. These techniques require very precise control over all operating parameters, do not produce an effective separation, and are too sensitive to utilize in a production environment. Furthermore, the non-conductive silicon carbide grains, in which impurities compensate one another electrically, would be lost the silica fraction.
Other methods for separating silicon carbide furnace materials, which have been considered in the prior art, include shot blasting or tumbling off the more friable outer layer, or abrading off the outer layer with a wire brush or similar cutter.